The present invention relates to an MRI (magnetic resonance imaging) apparatus, and more particularly to an MRI apparatus that can reduce ghost artifacts due to a Maxwell term phase error caused by a data acquisition read gradient.
FIG. 6 shows an exemplary pulse sequence in accordance with an EPI (echo planar imaging) technique.
In this pulse sequence, an excitation pulse RF90 and a slice gradient SG90 are applied. Subsequently, a phase encoding gradient Pn1 is applied. Next, an inversion RF pulse RF180 and a slice gradient SG180 are applied. Then, an alternating data acquisition read gradient r1, . . . , rI (I=4 in FIG. 6) having positive and negative polarities is consecutively applied, and at the same time, phase encoding gradients P2, . . . , PI are applied to sample 1st through I-th echoes e1-eI synchronously with sequential focusing of the 1st through I-th echoes, thereby collecting data dn1, . . . , dnI corresponding to the echoes e1-eI. This sequence is repeated for n=1, . . . , N, and data d11-dNI filling out a k-space are collected. This is referred to as an N-shotxc2x7I-echo process.
FIG. 7 is a schematic diagram illustrating a trajectory of collecting the data d11-dNI in a k-space KS, wherein N=4 and I=4.
When the k-space KS is divided into 1st through Nxc2x7I-th rows (16 rows in FIG. 6) in the direction of a phase encoding axis, phase encodings Pn1, P2, . . . , PI are applied so that data dni for an (n+(ixe2x88x921) N)-th row is collected by an i-th echo in an n-th shot.
Referring to FIG. 8, the k-space KS can be sequentially divided into a 1st echo block, which is filled with data dn1 acquired from a 1st echo in each shot, through an I-th echo block (I=4 in FIG. 6), which is filled with data dni acquired from an I-th echo in each shot.
FIG. 9 is a diagram illustrating a phase error due to magnetic field inhomogeneity of a magnet.
The phase error due to magnetic field inhomogeneity increases in proportion to the time period from the excitation pulse RF90, as indicated by a magnetic field inhomogeneity phase error characteristic line.
If the time period from the excitation pulse RF90 to the beginning of application of the data acquisition read gradient is the same among all the shots, all the data dni corresponding to i-th echoes ei have a phase error magnitude of Ui. Accordingly, the phase error exhibits a large stepped difference between the adjacent echo blocks, resulting in ghost artifacts.
Therefore, as shown in FIG. 10, the time period from the excitation pulse RF90 to the beginning of application of the data acquisition read gradient is sequentially delayed from a 2nd shot to an N-th shot by a delay time of 1/N an echo space (an echo space=a time period between the adjacent echoes=a time width of a read gradient corresponding to one echo). This technique is referred to as echo shift. The magnitude of phase error now changes linearly in the direction of the phase encoding axis in the k-space KS and does not exhibit the large stepped difference in the phase error between the adjacent echo blocks. The ghost artifacts can thus be reduced.
FIG. 12 shows an exemplary pulse sequence in accordance with a GRASE (gradient and spin echo) technique.
In this pulse sequence, an excitation pulse RF90 and a slice gradient SG90 are applied. Subsequently, a read gradient r0 is applied. Next, a j-th inversion RF pulse RF180_i (j=1, . . . , J. In FIG. 12, J=3) and a slice gradient SG180 are applied. Then, an alternating data acquisition read gradient rj1, . . . , rjI (I=3 in FIG. 12) having positive and negative polarities is consecutively applied, and at the same time, phase encoding gradients pj1, . . . , pjI are applied to sample 1st through I-th echoes for the j-th inversion RF pulse ej1-ejI synchronously with sequential focusing of the 1st through I-th echoes, thereby collecting data dnj1, . . . , dnjI corresponding to the echoes ej1-ejI. This sequence is repeated for n=1, . . . , N, and data d111-dNJI filling out a k-space are collected.
FIG. 13 is a schematic diagram illustrating a trajectory of collecting the data d111-dNJI in a k-space KS, wherein N=2, J=3 and I=3.
When the k-space KS is divided into 1st through Nxc2x7Jxc2x7I-th rows (18 rows in FIG. 13) in the direction of a phase encoding axis, a phase encoding pji for an n-th shot is applied so that data dnji for an (n+(ixe2x88x921) N+(ixe2x88x921) Nxc2x7J)-th row is collected by an i-th echo for a j-th inversion pulse in an n-th shot.
Also in the GRASE pulse sequence shown in FIGS. 12 and 13, ghost artifacts can be reduced by applying echo shift similarly to the EPI technique.
In the past, ghost artifacts due to a phase error caused by magnetic field inhomogeneity are thus reduced by the echo shift.
However, mere echo shift cannot fully reduce ghost artifacts because the conventional techniques have not accounted for ghost artifacts due to a phase error from a Maxwell term (which will be described in more detail later) caused by a data acquisition read gradient.
It is an object of the present invention to provide an MRI apparatus that can reduce ghost artifacts due to a Maxwell term phase error caused by a data acquisition read gradient.
In accordance with a first aspect of the invention, there is provided an MRI apparatus comprising pulse sequence generating means for generating a pulse sequence for collecting data, data collecting means for executing the generated pulse sequence to collect data, and image producing means for reconstructing an image from the collected data, wherein the pulse sequence generating means generates a pulse sequence for an n-th shot so that the following conditions are satisfied: (1) when a k-space is divided into 1st through Nxc2x7I-th rows (wherein N and I are natural numbers not less than 2) in the direction of a phase encoding axis, repeating for N shots a pulse sequence which applies a data acquisition read gradient while inverting the gradient to focus I echoes per inversion RF pulse, and collecting data for filling out the k-space; and (2) appending an n-th Maxwell term correction read pulse before an inversion RF pulse in an n-th shot (wherein n=1xe2x88x92N), the n-th Maxwell term correction read pulse having a waveform whose time integral value is zero, and giving a bias phase error such that a Maxwell term phase error which is caused by the data acquisition read gradient and contained in the data filling out the k-space smoothly varies from the 1st row to the Nxc2x7I-th row in the direction of the phase encoding axis (including, in a case of a shot taken as a reference, appending no Maxwell term correction read pulse only to that shot).
In other words, the present invention provides an MR imaging method for, when a k-space is divided into 1st through Nxc2x7I-th rows (wherein N and I are natural numbers not less than 2) in the direction of a phase encoding axis, repeating for N shots a pulse sequence which applies a data acquisition read gradient while inverting the gradient to focus I echoes per inversion RF pulse, and collecting data for filling out the k-space, the method comprising the step of: appending an n-th Maxwell term correction read pulse before an inversion RF pulse in an n-th shot (wherein n=1xe2x88x92N), the n-th Maxwell term correction read pulse having a waveform whose time integral value is zero, and giving a bias phase error such that a Maxwell term phase error which is caused by the data acquisition read gradient and contained in the data filling out the k-space smoothly varies from the 1st row to the Nxc2x7I-th row in the direction of the phase encoding axis (including, in a case of a shot taken as a reference, appending no Maxwell term correction read pulse only to that shot).
If a main magnet field is B0, and linear gradient magnetic fields in the X-, Y- and Z-directions are Gx, Gy and Gz, a magnetic field Bz(x, y, z, t) at a point (x, y, z) and at a time (t) can ideally be given by:
Bz(x,y,z,t)=B0+Gx(t)xc2x7x+Gy(t)xc2x7y+Gz(t)xc2x7z.
In practice, the magnet field Bz(x, y, z, t) contains an additional term BM(x, y, z, t) so that a Maxwell equation is satisfied:
Bz(x,y,z,t)=B0+Gx(t)xc2x7x+Gy(t)xc2x7y+Gz(t)xc2x7z+BM(x,y,z,t).
The additional term BM is referred to as a Maxwell term and is given by:                                           B            M                    =                                                                      G                  z                  2                                ⁢                                  x                  2                                                            8                ⁢                                  B                  0                                                      +                                                            G                  z                  2                                ⁢                                  y                  2                                                            8                ⁢                                  B                  0                                                      +                                                            (                                                            G                      x                      2                                        +                                          G                      y                      2                                                        )                                ⁢                                  z                  2                                                            2                ⁢                                  B                  0                                                      -                                                            G                  y                                ⁢                                  G                  z                                ⁢                yz                                            2                ⁢                                  B                  0                                                      -                                                            G                  x                                ⁢                                  G                  z                                ⁢                xz                                            2                ⁢                                  B                  0                                                                    ,                            (        1        )            
wherein the Z-direction is defined as a main magnet field direction.
Assume that the slice axis corresponds to the Y-direction, the read axis to the X-direction and the phase encoding axis to the Z-direction. Since the third term in Equation (1) is predominant, a phase error xcfx86M from the Maxwell term BM caused by the data acquisition read gradient is given by:                                           φ            M                    =                                                    γ                ⁢                                  xe2x80x83                                ⁢                                  Z                  2                                                            2                ⁢                                  B                  0                                                      ⁢                                          ∫                0                τ                            ⁢                                                                    G                    x                    2                                    ⁡                                      (                    t                    )                                                  ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                                    ,                            (        2        )            
wherein t=0 at the beginning time of application of the read gradient.
As demonstrated by Equation (2), the phase error from the Maxwell term caused by the data acquisition read gradient increases in proportion to the time period from the beginning of application of the data acquisition read gradient. This is indicated by a Maxwell term phase error characteristic line in the EPI technique, shown in FIG. 11. Since the beginning time of application of the read gradient is shifted among the shots by echo shift, the Maxwell term phase error characteristic line is also shifted among the shots.
As can be seen from FIG. 11, the Maxwell term phase error caused by a data acquisition read gradient for data dni corresponding to an i-th echo ei in an n-th shot has a magnitude of Mi in an i-th echo block. Hence, the phase error exhibits a large stepped difference between the adjacent echo blocks, resulting in ghost artifacts.
Therefore, according to the MRI apparatus of the first aspect, an n-th Maxwell term correction pulse is appended before an inversion RF pulse in an n-th shot. The n-th Maxwell term correction pulse does not affect the phase encoding amount because of its waveform having a time integral value of zero. The Maxwell term phase error still has a fixed amount. Moreover, the amount gives a bias phase error such that a Maxwell term phase error which is caused by a data acquisition read gradient and contained in data filling out the k-space smoothly varies from a 1st row to an Nxc2x7I-th row in the direction of a phase encoding axis. Thus, the Maxwell term phase error caused by the data acquisition read gradient does not exhibit a large stepped difference between the adjacent echo blocks, and ghost artifacts can be reduced.
In accordance with a second aspect of the invention, there is provided an MRI apparatus comprising pulse sequence generating means for generating a pulse sequence for collecting data, data collecting means for executing the generated pulse sequence to collect data, and image producing means for reconstructing an image from the collected data, wherein the pulse sequence generating means generates a pulse sequence for an n-th shot so that the following conditions are satisfied: (1) when a k-space is divided into 1st through Nxc2x7Jxc2x7I-th rows (wherein N is a natural number not less than 1, and J and I are natural numbers not less than 2) in the direction of a phase encoding axis, repeating for N shots a pulse sequence which applies J inversion RF pulses per excitation RF pulse and applies a data acquisition read gradient while inverting the gradient to focus I echoes per inversion RF pulse, and collecting data for filling out the k-space; and (2) appending an nj-th Maxwell term correction read pulse before or after a j-th inversion RF pulse (wherein j=1xe2x88x92J) in an n-th shot (wherein n=1xe2x88x92N), the nj-th Maxwell term correction read pulse having a waveform whose time integral value is zero, and giving a bias phase error such that a Maxwell term phase error which is caused by the data acquisition read gradient and contained in the data filling out the k-space smoothly varies from the 1st row to the Nxc2x7Jxc2x7I-th row in the direction of the phase encoding axis (including, in a case of a shot taken as a reference, appending no Maxwell term correction read pulse only to that shot).
In other words, the present invention provides an MR imaging method for, when a k-space is divided into 1st through Nxc2x7Jxc2x7I-th rows (wherein N is a natural number not less than 1, and J and I are natural numbers not less than 2) in the direction of a phase encoding axis, repeating for N shots a pulse sequence which applies J inversion RF pulses per excitation RF pulse and applies a data acquisition read gradient while inverting the gradient to focus I echoes per inversion RF pulse, and collecting data for filling out the k-space, the method comprising the step of: appending an nj-th Maxwell term correction read pulse before or after a j-th inversion RF pulse (wherein j=1xe2x88x92J) in an n-th shot (wherein n=1xe2x88x92N), the nj-th Maxwell term correction read pulse having a waveform whose time integral value is zero, and giving a bias phase error such that a Maxwell term phase error which is caused by the data acquisition read gradient and contained in the data filling out the k-space smoothly varies from the 1st row to the Nxc2x7Jxc2x7I-th row in the direction of the phase encoding axis (including, in a case of a shot taken as a reference, appending no Maxwell term correction read pulse only to that shot).
FIG. 14 illustrates a Maxwell term phase error characteristic in the GRASE technique, wherein N=1, J=3 and I=3.
As can be seen from FIG. 14, the Maxwell term phase error caused by a data acquisition read gradient for data dji corresponding to an i-th echo eji for a j-th inversion pulse RF180_i in an n-th shot has a magnitude of Mi in an i-th echo block. Hence, the phase error exhibits a large stepped difference between the adjacent echo blocks, resulting in ghost artifacts.
Therefore, according to the MRI apparatus of the second aspect, an nj-th Maxwell term correction pulse is appended before or after a j-th inversion RF pulse in an n-th shot. The nj-th Maxwell term correction pulse does not affect the phase encoding amount because of its waveform having a time integral value of zero. The Maxwell term phase error still has a fixed amount. Moreover, the amount gives a bias phase error such that a Maxwell term phase error which is caused by a data acquisition read gradient and contained in data filling out the k-space smoothly varies from a 1st row to an Nxc2x7Jxc2x7I-th row in the direction of a phase encoding axis. Thus, the Maxwell term phase error caused by the data acquisition read gradient does not exhibit a large stepped difference between the adjacent echo blocks, and ghost artifacts can be reduced.
In accordance with a third aspect of the invention, there is provided the MRI apparatus as described regarding the first or second aspect, wherein the pulse sequence generating means generates a pulse sequence for an n-th shot so that the following condition is also satisfied: (3) appending the Maxwell term correction read pulse such that a Maxwell term phase error which is caused by the data acquisition read gradient and contained in data of, or proximate to, a DC component in the k-space is equal to, or proximate to, zero.
In the MRI apparatus of the third aspect, since a Maxwell term phase error which is caused by a data acquisition read gradient and contained in data of, or proximate to, a DC component, the most important to image reconstruction, in the k-space is equal to, or proximate to, zero, the effect of the Maxwell term phase error caused by the data acquisition read gradient on an image can be reduced to a minimum.
Thus, the MRI apparatus of the present invention can reduce ghost artifacts due to a Maxwell term phase error caused by a data acquisition read gradient.